1. A Project Report on
Mapping of Focal Mechanism of Earthquakes in
Indian Region
By
TAICHENGMONG RAJKUMAR
From
Department of Applied Geophysics
Indian School of Mines, Dhanbad
Carried out under
Student Programme for Advancement in Research Knowledge
(SPARK) at
Council of Scientific & Industrial Research
CENTRE FOR MATHEMATICAL MODELLING AND
COMPUTATIONAL SIMULATION (CSIR-C-MMACS)
NAL BELUR CAMPUS, BANGALORE-560037
Under the guidance of
Dr. Imtiyaz Ahmed Parvez
Principal Scientist, C-MMACS, Belur Campus (NAL), Bangalore-560037
Department of Applied Geophysics
Indian School of Mines, Dhanbad-826004
2. Date: ………………………
CERTIFICATE
This is to certify that the project entitled “Mapping of Focal Mechanism of
Earthquakes in Indian Region” submitted by Taichengmong Rajkumar to Indian
School of Mines, Dhanbad in partial fulfillment of the requirement for the award of
the degree for Int. Master of Science and Technology is a record of bonafide work
carried out by him under my supervision and guidance at CSIR CENTRE FOR
MATHEMATICAL AND COMOUTER SIMULATION (C-MMACS),
NATIONAL AEROSPACE LABORATORIES (NAL), BANGALORE. It is also
certified that the project work has not been submitted for any purpose elsewhere, in
part or full.
Dr. Imtiyaz Ahmed Parvez Signature of the Convenor, SPARK
C-MMACS
Project Guide
CSIR Centre for Mathematical Modelling and
Computer Simulation
(Council of Scientific & Industrial Research)
NAL Belur Campus, Bangalore-560037, India
3. ACKNOWLEDGEMENT
I am very grateful to my guide Dr Imtiyaz Ahmed Parvez who helped me a lot during
the whole project. He has been very kind to me and shared his knowledge with me. I
would also like to thank Dr Anil Kumar, Convenor of SPARK for being so patient to
us. I also convey my heartiest thanks to Stella madam for her help and support. I
would like to thank Prof. Shalivahan sir, HOD, Department of Applied Geophysics,
Indian School of Mines, Dhanbad who has always been a support to us.
Also I would like to thank my fellow summer interns for helping me throughout these
two months.
4. INTRODUCTION
Earthquakes occur on faults. A fault is a thin zone of crushed rock separating blocks
of the earth's crust. When an earthquake occurs on one of these faults, the rock on
one side of the fault slips with respect to the other. Faults can be centimeters to
thousands of kilometers long. The fault surface can be vertical, horizontal, or at some
angle to the surface of the earth. Faults can extend deep into the earth and may or
may not extend up to the earth's surface.
Based on the nature of relative movement along the fault it can be classified into three
types:
(i) Thrust fault
(ii) Normal fault
(iii) Strike-slip fault
The block above the fault plane is called the hanging wall and that below the fault
plane is footwall. Dip is the angle between the horizontal surface and the plane of the
fault; hade is compliment of the dip. A standard nomenclature rake has evolved for
describing slip direction. The actual motion of the two blocks on either side of the
fault plane is defined by a slip vector which can have any orientation on the fault
plane. The direction of slip vector is given by the angle of slip or rake (λ). It is
measured in the plane of the fault from the strike direction to the slip vector showing
the motion of the hanging wall relative to the footwall.
Thrust Fault:
A thrust fault is a fault along which the hanging wall (upper side of the fault) has
moved up relative to the foot-wall. The thrust is one that dips less than 45º and an
5. over thrust that dips less than 10º. In pure thrust-faulting the slip vector is parallel to
the dip direction and it is upward, so λ= 90º. Thrust faulting involves crustal
shortening and implies compression.
Normal Fault:
A normal fault is a fault along which hanging wall has moved relatively downward.
In pure normal faulting the slip vector is also parallel to the dip direction of the fault
plane but it is downward i.e λ = -90º (270º). Normal faulting involves lengthening of
the crust and implies tension. There are many possibilities concerning the actual
movement; the footwall may remain stationary and the hanging wall goes down; or
the hanging wall remains stationary and the footwall goes up, or both blocks move
down but the hanging wall moves more than the footwall, or both blocks move up;
but the footwall moves more than the hanging wall. Some geologists use the term
gravity fault in preference to normal fault.
Strike-slip Fault:
A strike slip fault is a fault along which displacement has been essentially parallel to
the strike of the fault, that is the dip-slip component is less or negligible (λ = 0 or
180º). For λ = 0, the hanging wall moves to the right so that the opposite wall, faced
by an observer, moves relatively to the left. This is called left-lateral slip or sinistral
fault. When λ= 180º, the hanging wall moves to the left and the opposite wall faced
by an observer moves relatively to the right. This is right-lateral slip or dextral fault.
In general λ will have a value different than these special cases and the motion is then
called oblique slip.
6. FOCAL MECHANISM
Seismologists refer to the direction of slip in an earthquake and the orientation of the
fault on which it occurs as the focal mechanism. They use information from
seismograms to calculate the focal mechanism and typically display it on maps as
“beach ball” symbol. This symbol is the projection on a horizontal plane of the lower
7. half of an imaginary, spherical shell (focal sphere) surrounding the earthquake source
(A). A line is scribed when the fault plane intersects the shell. The stress-field
orientation at the time of rupture governs the direction of slip on the fault plane, and
the beach ball also depicts this stress orientation. In this schematic, the gray quadrants
contain the tension axis (T), which reflects the minimum compressive stress direction,
and the white quadrants contain the pressure axis (P), which reflects the maximum
compressive stress direction. The computed focal mechanisms show only the P and
T axes and do not use shading.
These focal mechanisms are computed using a method that attempts to find the best
fit to the direction of P-first motions observed at each station. For a double-couple
source mechanism (or only shear motion on the fault plane), the compression first-
motions should lie only in the quadrant containing tension axis, and the dilatation
first-motions should lie only in the quadrant containing the pressure axis. However,
first-motion observation will frequently be in the wrong quadrant. This occurs
because a) the algorithm assigned an incorrect first-motion direction because the
signal was not impulsive, b) the earthquake velocity model, and hence, the earthquake
location is incorrect, so that the computed position of the first-motion observation on
the focal sphere (or ray azimuth and angle of incidence with respect to vertical) is
incorrect, or c) the seismometer is mis-wired, so that “up” is “down”. The latter
explanation is not a common occurrence. For mechanisms computed using only first
–motion directions, these incorrect first-motion observations may greatly affect the
computed focal mechanism parameters. Depending on the distributed and quality of
first-motion data, more than one focal mechanism solution may fit the data equally
well.
For mechanism calculated from first-motion directions as well as some methods that
model waveforms, there is an ambiguity in identifying the fault plane on which slip
occurred form the orthogonal, mathematically equivalent, auxiliary plane. We
illustrate this ambiguity with four examples (B). The block diagrams adjacent to each
8. focal mechanism illustrate the two possible types of fault motion that the focal
mechanism could represent. Note that the view angle is 30-degree to the left of and
above each diagram. The ambiguity may sometimes be resolved by computing the
two fault-plane orientation to the alignment of small earthquakes and aftershocks.
The first three examples describe fault motion that is purely horizontal (strike slip) or
vertical (normal or reverse). The oblique-reverse mechanism illustrate that slip may
also have components of horizontal and vertical motion.
9. Seismic Moment Tensor:
The seismic moment tensor M can be written in the form of 3x3 matrix as
where each component represents one of the nine possible force couples. A force
couple consists of two forces acting together. M12 consists of two forces of magnitude
f, separated by a distance d along 2-axis, that act in opposite directions along 1. The
magnitude of M12 = fd, which has unit of Nm. In the case of M11, two forces of
magnitude f are separated by a distance d along 1-axis, and act in opposite directions
along 1 axis. This type of couple is sometimes referred to as vector dipole. There will
be no torque in case of M11. The moment tensor can be described in terms of three
orthogonal axes: P (for pressure; a compressive axis), T (for tension), N (for null).
Fault surface along which the earthquake was generated is 45 degree from the P and
T axes, and contain the N axis. For any moment tensor there will be two nodal planes.
One nodal plane is perpendicular to other nodal plane and intersects along the N axis.
One of the planes is the fault surface and other is called as auxiliary plane.
10. Harvard CMT Solution:
The Global Centroid-Moment-Tensor (CMT) Project is overseen by Principal
Investigator Göran Ekström and Co-Principal Investigator Meredith Nettles at the
Lamont-Doherty Earth Observatory (LDEO) of Columbia University. The project
was founded by Adam Dziewonski at Harvard University (USA) and operated there
as the Harvard CMT Project from 1982-2006, led first by Prof. Dziewonski and later
by Prof. Ekström. During the summer of 2006, the activities of the CMT Project
moved with Prof. Ekström to LDEO. This research effort is moving forward under
the name "The Global CMT Project". The main dissemination point for information
and results from the project is the web site www.globalcmt.org.
The CMT project has been continuously funded by the National Science Foundation
since its inception, and is currently supported by award EAR-0824694.
Focal Mechanism solutions in Indian Region:
For focal mechanism solutions, I have used CMT HARVARD catalog. For this I have
obtained focal mechanism during 01-01-1976 to 31-05-2015. I have used more than
140 focal mechanism solutions of earthquakes. The corresponding epicentres are
located inside a quadrangle from 8º N to 38º N in latitude and 68º E to 98º E in
longitude. The magnitude range is chosen from 5.5 to 8.
Data for plotting Focal Mechanism:
Lon lat depth mrr mtt mpp mrt mrp mtp iexp
88.14 33.00 10 -2.05 -1.74 3.80 -0.62 -3.73 0.98 25 88.4 34.2
89.16 30.69 10 -3.48 -0.82 4.31 -0.12 0.82 0.33 25 89.4 31
89.05 27.42 44 -0.24 -1.83 2.07 1.51 -1.12 -1.26 25 89.4 27
70.99 36.80 228 5.47 -6.20 0.73 1.03 1.30 -0.71 25 69.8 34.5
91.28 34.21 10 -0.04 -0.63 0.66 -0.12 0.20 1.23 25 90.5 33.6
73.48 35.22 10 1.81 -1.57 -0.25 -0.46 0.44 0.71 25 74.2 36.3
16. 70.44 20.98 12 0.02 -4.45 4.42 2.98 0.53 1.98 23 69.8 20.5
73.75 34.88 18 2.94 -0.51 -2.43 -0.62 2.50 -2.17 23 74.4 34
75.60 33.02 20 2.23 -1.08 -1.14 2.17 -1.21 1.23 24 75.3 33
75.95 33.09 22 5.26 -3.62 -1.64 2.32 0.94 3.25 23 76.8 32.6
75.71 33.10 26 3.58 -2.75 -0.82 3.38 0.00 1.80 23 76 32.6
The focal mechanism solutions are plotted in the map using GMT software and
various zones have been mapped from the clusters of similar focal mechanism. The
radius of the beach balls shown in figure 1 represents the size of the earthquake.
Bigger the size of beach balls corresponds to higher magnitudes.
Different Zones and their characteristic faults:
Zone 1: This zone covers the earthquakes occurred at Hindu Kush area. These are
mostly Thrust faults.
Zone 2: It covers the earthquakes occurred at the boundary of Jammu and Kashmir
and China. These are Strike-Slip faults.
Zone 3: It covers the Tibet region. These are Strike-Slip faults.
Zone 4: It covers the Himalayas. These are Strike Slip faults.
Zone 5: It covers Jammu and Kashmir. These are Thrust faults.
Zone 6: It covers Uttarakhand and Nepal. These are Thrust faults.
Zone 7: It covers the boundary of Nepal and Himalayas. These are Normal faults.
Zone 8: It covers the Himalayas. These are Normal faults.
Zone 9: It covers the Tibet region. These are Strike-Slip faults.
17. Zone 10: It covers the Tibet region. These are Thrust faults.
Zone 11: It covers the earthquakes occurred in Arunachal Pradesh. These are Thrust
faults.
Zone 12: It covers the boundary of NE India and Myanmar. Here the fault type is
Thrust fault.
Zone 13: It covers the earthquakes occurred in Myanmar. These are Strike-Slip
faults.
Zone 14: This zone covers the earthquakes of Andaman and Nicobar region. These
are Thrust faults.
Zone 15: It covers the Andaman and Nicobar region. The fault type is Thrust fault.
Zone 16: It covers the Andaman and Nicobar region. These are Strike-Slip faults.
Zone 17: It covers the Andaman and Nicobar region. These are Thrust faults.
Zone 18: It covers Andaman and Nicobar region. These are Normal faults.
Zone 19: It covers the earthquakes occurred in Madhya Pradesh and Western
Maharashtra. These are Thrust faults.
Zone 20: This zone covers the earthquakes of Gujrat and Rajasthan. These are Strike-
Slip faults.
Zone 21: It covers Sikkim and Bhutan. The fault type is Strike-Slip fault.
19. Another map is plotted showing the Pressure and Tension axes.
Pressure axis •
Tension axis •
Fig 2: Focal Mechanisms with Pressure and Tension axes
20. DISCUSSION AND CONCLUION
The upper mantle beneath the mountainous Hindu Kush region of northeastern
Afghanistan is the site of a tectonically complex area. Although it is not clearly
associated with any island arc system, this region is perhaps the most active zone of
intermediate depth (70-300km) earthquakes in the world. The region is therefore
interesting, since it provides a setting for examining deep-seated tectonic processes
in a collision zone as well as allowing a study of intermediate depth seismicity as
phenomenon in it. Because of its proximity to the Eurasian- Indian plate boundary
the Hindu- Kush seismic zone is believed to be grossly related to the convergence of
the Indian and Eurasian sub-continents. We have two different types of fault plane
solution in Hindu Kush region. Fault planes with solutions that in the west have
westward plunging T axes and in the east have eastward or vertically plunging T axes.
We infer that the configuration of the Hindu Kush seismic zone could possibly be the
result of subduction of oceanic lithosphere from two separate, small basins in
opposite directions.
The thrust type faulting is prevalent in the Nepal region and confirmed that the under
thrusting of Indian plate towards the north along the Himalayan arc has occurred. It
is observed thrust and strike slip components and inferred that there is under thrusting
of Indian plate towards southwest. Some fault-plane solutions of Tibetan region and
observed that there is a presence of a combination of normal and strike-slip faulting
with T-axes trending approximately east-west. There is a coherent under thrusting of
Indian plate beneath the Lesser Himalaya in the eastern half of the arc. They inferred
that slip vectors are locally perpendicular to the Himalayan mountain range with very
gentle plunge in the eastern section and more steeply in the western section. It is
apparent that the compressive stress is acting in N-S to NE–SW directions which are
21. approximately perpendicular to the major trend of the Himalaya. It also reveals that
the earthquake generation process in the region is due to the north–northeast
compressive stress exerted by the Indian plate to the Tibetan plate. However, the
plunges of P-axis of a few events show compression from approximately northwest
direction.
In the NE region near Nagaland and Manipur we have found from the focal
mechanism and orientation of P and T axes that the CMF, a geologically older thrust
fault, accommodates motion through dextral strike–slip manner, which is a part of
relative plate motion between the India and Sunda plates. Therefore, CMF is the
present-day active deformation front or plate boundary fault between the India and
Burma plates.
In Andaman region the strike slip events can hardly be designated as inter-plate
events due to (i) the their deep occurrence well within the underlying Indian plate
zone, and (ii) because of their nearly vertical fault planes oriented NW and NE
respectively, contrasting with the shallow dipping NS trending decollement plane in
the India-Burma subduction zone. Earthquakes with a similar mechanism do occur in
the Andaman arc region, except that they are associated with the transform faulting
in the Andaman sea, the Sumatran fault zone to the east or the NS trending right-
lateral strike-slip faulting along the Sagaing fault zone farther east. Hence, these
strike-slip events are distinct from the above mechanisms and are seen to be
associated with intra-plate deformation in the subducting Indian plate.
The focal mechanisms of earthquakes in Gujrat region were the result of movement
in a fault at shallow depth, caused by the stress within the Indian tectonic plate
pushing northward into the Eurasian plate. They are also called intraplate earthquakes
as they occur in the interior of a tectonic plate different that those Himalayas
earthquakes that occur at the plate boundary.
22. REFERENCES
• Principles of Seismology: Agustίn Udίas
• Lecture Notes Focussing on MICROEARTHQUAKE INVESTIGATIONS:
NGRI, Hyderabad
• Discourse on Seismotectonics of Nepal Himalaya and Vicinity: Appraisal to
Earthquake Hazard by D. Shanker1, Harihar Paudyal, H. N. Singh
